How to download and create boot disk

Ok, so first you need to download it here (this is from apple’s website)

https://apps.apple.com/us/app/macos-high-sierra/id1246284741?mt=12

then, type in the terminal:

sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Or copy it from here:https://support.apple.com/en-gb/HT201372

you will need a USB when you do this part and replace the part of the command that says: MyVolume with the name of your USB. WARNING ANYTHING ON THE USB WILL BE DELETED. Then just reboot and install.

How to get old versions of macOS and also https://support.apple.com/en-us/HT201372